1. Field of the Invention
The present invention relates to an image reader which provides a mechanism for varying the reading magnification optically.
2. Description of Related Art
As is well known, in the image reader mentioned above, a focusing lens (projection lens) is arranged on the light path from a document to an image sensor and an image of a document is focused on the image reader at a predetermined reduction ratio. Upon varying the reading magnification, the focusing lens is moved along the optical axis thereof and the image sensor is also moved relative to the focusing lens so as to focus the image of a document thereon. Respective moving amounts of the focusing lens and the image sensor are calculated according to equation defined from the known optical relation.
In the image reader of this type, there is such a possibility that the center position of a document on the image sensor is shifted from that obtained at the equal magnification when the reading magnification is varied from the equal magnification. If the shift amount on the image sensor is about 0.1 mm, it corresponds to a shift amount of several times of 0.1 mm on a document since the optical system of the image reader is comprised of a reduction optical system. Such a low mechanical accuracy makes the practical use of the image reader of this type difficult.
Factors which cause such an mechanical error are considered as follows;
(a) a guide axis for guiding the focusing lens is slightly inclined to the optical path along which the focusing lens is to be moved. PA1 (b) a guide axis for guiding the image sensor is slightly inclined to the optical axis of the focusing lens.
Conventionally, the center position of a document image is fixed at the center position of the CCD image sensor and, due to this, possible shift of the center position of a document image which may be caused by varying the reading magnification is not compensated at all. As the result of that, such cases that respective center positions of document images outputted at different magnifications do not coincide with each other are happened frequently.